A goal of semiconductor device manufacturing is to make integrated circuits as small as possible. As devices become smaller, low-k dielectrics are needed to reduce parasitic capacitance and RC switching delay. These materials are particularly useful as intermetal dielectrics, or IMDs, and as interlayer dielectrics, or ILDs.
Low-k dielectric materials refer to those insulating materials that have a dielectric constant lower than that of silicon dioxide, or less than about 4. One example of a low-k material is fluorine-doped silicon dioxide, or fluorosilicate glass (FSG). Another widely used material is a carbon-doped oxide or organosilicate glass (OSG). OSG films typically comprise SiwCxOyHz, wherein the tetravalent silicon may have a variety of organic group substitutions. A commonly used substitution creates methyl silsesquioxane (MSQ), wherein a methyl group creates a SiCH3 bond in place of a SiO bond.
There are several approaches known in the art for reducing the k-value of dielectric films. These include decreasing the film density, reducing the film ionization, and reducing the film polarization. Reduced ionization and reduced polarization are a common feature of carbon-containing, low-k dielectric films. For example, the Si—CH3 bond is less polar than the Si—O bond. Its tendency to ionize is less as well. Engineering of the organic functionality in low-k films is an important tool for optimizing the properties of these materials.
A major drawback with low-k dielectrics is that they are susceptible to damage from plasma etching and ashing processes used in device fabrication. Such plasma processes include etching, including etching of the low-k film, removing photoresist masking material, and depositing layers in plasma-enhanced chemical vapor deposition (PECVD) processes. In etch and ash processing, low-k materials frequently suffer from carbon depletion at the surface exposed to the plasmas. In certain etch and ash processes, the damage may also extend into the bulk as well. Upon subsequent exposure to air, these damaged surfaces react with moisture to form silanol groups (≡Si—OH) at free Si sites, if these sites are not already occupied by oxygen during the etch or ash process. The silanol group is known in the art to increase the dielectric constant of the low-k dielectric material. It is also known that the damaged low-k dielectric material is vulnerable to chemical attack during exposure to wet chemical cleanups, which results in significant critical dimension (CD) loss of low-k dielectric film insulating structures. Similar effects are believed to occur in other low-k dielectric materials with silicon-hydrocarbon bonds that are converted to silanol when exposed to oxidizing or reducing plasmas.
Recognizing the need to overcome these drawbacks, semiconductor manufacturers have developed methods to repair damaged low-k dielectric layers. One conventional repair method includes thermal annealing of the low-k dielectric film. However, thermal annealing raises concerns regarding other problems such as thermally induced copper migration. Thermal annealing is also disfavored in that it requires economically unfavorable processing times and equipment costs. Finally, plasma-damaged low-k dielectric films that are annealed according to conventional processes are vulnerable to the re-adsorption of moisture and reformation of silanol.
Another conventional approach includes treating the damaged insulation layer with a silylation agent such as hexamethyldisilazane (HMDS). In this method, a trimethlysilane group replaces the hydrogen of the silanol group.
Silylation effectively eliminates the silanol functional group. However, it suffers from the limitation of being able to replace hydrogen with only a silyl group, in this example, a trimethylsilyl group. This shortcoming limits an IC manufacturer's ability to engineer the density, polarization, and ionization properties of the low-k dielectric film.
Semiconductor manufacturers need a method for repairing carbon depletion in low-k dielectrics that is amenable to a wide category of organic compounds.